The present invention relates to an interstage casing for a pump made of sheet metal, and more particularly to an interstage casing for a pump made of sheet metal and pressed into shape for use in a multistage centrifugal pump.
Conventionally, there is known an interstage casing for a pump in which a casing is formed of sheet metal such as a stainless steel plate and manufactured by press working the sheet metal.
This type of interstage casing is shown in FIG. 5 of the accompanying drawings. FIG. 5 shows the interstage casing housing return blades therein. The interstage casing shown in FIG. 5 is constructed as follows. As shown in FIG. 6 (a) of the accompanying drawings, a metal sheet such as a stainless steel plate is pressed into a cylindrical receptacle-like structure comprising a cylindrical side wall 1 and a bottom wall (or casing end wall) 2 on an outlet end thereof. Then, as shown in FIG. 6 (b), an open inlet end 3 of the cylindrical side wall 1 is bent radially inwardly. Thereafter, as shown in FIG. 5, a return blade 4 and other parts are welded to the inner surface of the bottom wall 2. Then, the outer peripheral surface of the bottom wall 2 is machined to form a radially outer surface 2a and a radial end surface 2b which lie perpendicularly to each other, providing the male member of a spigot joint for fitting engagement with the open inlet end of a subsequent interstage casing which houses an impeller for developing a higher head. The open inlet end 3 is also machined to form a radially inner surface 3a and a radial end surface 3b which lie perpendicularly to each other, providing the female member of a spigot joint for fitting engagement with the outer peripheral surface of the bottom wall of a preceding interstage casing which houses an impeller for developing a lower head. Incidentally, finish marks are used to represent machined portions.
The interstage casing of the above structure houses an impeller 5 that can be rotated by a shaft 6. A liner ring 7 is attached to the bottom wall 2.
FIG. 7 of the accompanying drawings illustrates a multistage centrifugal pump comprising a plurality of series-connected interstage casings 1, each having the structure shown in FIG. 5. As shown in FIG. 7, the interstage casings 1 are assembled together by a fastening band 8. The shaft 6 is rotatably supported by an upper bearing 9a and a lower bearing 9b. The multistage centrifugal pump includes a discharge casing 10, a suction casing 11, a valve body 12, a strainer 13, and a cable cover 14.
When the multistage centrifugal pump is in operation, the liquid to be pumped is drawn through the strainer 13 and its pressure is increased successively by the impellers 5. The pressure head of the liquid is restored as the liquid passes through each of the return blades 4. Finally, the liquid is discharged out of the pump through a discharge port defined in the discharge casing 10.
The interstage casing of pressed sheet metal shown in FIG. 5 maintains a desired degree of dimensional accuracy for diameters and heights through the machining of the four regions, i.e., the radially outer surface 2a, the radial end surface 2b, the radially inner surface 3a, and the radial end surface 3b. However, these surfaces have to be machined with utmost care because the pressed sheet metal is thin enough to be easily deformed by forces that are applied when the sheet metal is fastened for machining, or by stresses and heat that are developed when the steel sheet is machined.
Sealing between the interstage casings is achieved by liquid gaskets that are of relatively low reliability since no installation space is available between the interstage casings for O-rings that are widely used in pump casings made by casting. Consequently, the interstage casings with liquid gaskets are not suitable for use in applications that require the development of very high pressures or environments that should be kept free of liquid leaked from the pump.